The type of airless pump used in the present invention is shown and described in U.S. Pat. No. 4,035,107. The said patent also includes a relief valve of the type used in the present invention and is shown in FIG. 5 of the patent and described as unloading valve 22.
In a paint spraying operation, the painter usually starts spraying the top of a wall, for example, and continues spraying downwardly until the entire vertical portion of the wall is painted. At this time the painter closes the spray nozzle and directs the nozzle to the upper part of the wall adjacent the column that has been just painted. The nozzle is then opened. These steps are repeated throughout the entire painting job.
One of the requirements in this type of operation is the need for immediate full flow of paint through the nozzle when it is opened. If the pressure is reduced significantly or the flow is decreased each time the nozzle is repeatedly opened, time is lost and the painting is not uniform at the start and fishtailing occurs.
In the prior art, gasoline engines for driving airless paint spray pumps were speed adjusted before starting to spray. The speed adjustment was determined according to the requirement of the size of the spray nozzle to be used. The engine rpm may be as high as 4000 and the pressure may be 2500 psi. During the painting operation, when the nozzle was shut off, paint was relieved through a relatively small relief valve. While the paint was allowed to continuously flow through the relief valve at the set high rate, this heated the paint excessively. The heat caused a drying effect in the paint relieved to the paint source.
Another problem that occured in the above high speed pre-setting adjustment was that when the nozzle size had to be changed, the speed of the engine had to be readjusted. This required the painter to come down from a scaffold or roof, etc. to adjust the engine speed for the different size nozzle. The same kind of adjustment problem occurred when two nozzles were operated from one pump and one of the nozzles was shut off.
The basic problems were that there was no convenient way to slow the engine when the nozzle was frequently closed and then speeded up when the nozzle was reopened.